The use of ink jet printers for printing information on a recording media is well established. Printers employed for this purpose may be grouped into those that use a continuous stream of fluid drops and those that emit drops only when corresponding information is to be printed. The former group is generally known as continuous inkjet printers and the latter as drop-on-demand inkjet printers. The general principles of operation of both of these groups of printers are very well recorded. Drop-on-demand inkjet printers have become the predominant type of printer for use in home computing systems, while continuous inkjet systems have found a major application in industrial and professional environments.
Continuous inkjet printers typically have a print-head that incorporates a fluid supply system and a nozzle plate with one or more ink nozzles fed by the fluid supply system. Fluid streams are consequently jetted from the one or more ink nozzles. In order to create the ink drops, a drop generator is associated with the print-head. The drop generator influences the fluid streams within and just beyond the print-head by a variety of mechanisms discussed in the art. This is done at a frequency or multiple frequencies that forces these thread-like fluid streams to be broken up into corresponding continuous streams of drops at a point within the vicinity of the nozzle plate. Specific drops within these continuous streams of drops are then selected to be printed with or to not be printed with.
The means for selecting printing drops from non-printing drops within the continuous stream in drops have been well described in the art. One commonly used practice is that of electrostatically charging and electrostatically deflecting selected drops as described by Hansell in U.S. Pat. No. 1,941,001, and by Sweet et. al. in U.S. Pat. No. 3,373,437. In these patents, a charge electrode is positioned adjacent to a fluid stream at a point in which the corresponding continuous stream of drops forms. The function of the charge electrode is to selectively charge the fluid drops as the drops break off from the jet. This is possible because the jetted fluid has conductive properties. One or more electrostatic deflection plates positioned downstream from the charge electrodes deflect a charged fluid drop either into a gutter assembly or onto a recording media. For example, the drops to be guttered are charged and consequently deflected into the gutter assembly and those intended to print on the recording surface are not charged and continue un-deflected towards the recording surface. In some systems, this arrangement is reversed and the uncharged drops are guttered while the charged ones are ultimately printed. Electrostatic systems are advantageous in that they permit large drop deflections.
In electrostatic continuous inkjet systems in which such charging is required, various forms of charge electrodes have been described in the prior art for charging drops as they break off from fluid stream. Charge electrodes previously used in the art have typically comprised an electrically conductive material coated onto a nonconductive substrate. As disclosed by Loughren in U.S. Pat. No. 3,404,221, and by Sweet et. al. in U.S. Pat. No. 3,373,437, early charged elctrodes utilized cylindrically shaped hollow rings or tubes or U-shaped channels. However, the accurate placement of the tubes or channels into a support structure and then electrically connecting such devices to a signal source was both difficult and time consuming especially in multi-jet systems utilizing hundreds of individual streams of ink drops spaced only a few thousandths of an inch apart. Other charge electrode configurations have also included structures that partially enclose the fluid stream such as U or V-shaped electrodes.
Another example of charge electrodes was disclosed by Robertson in U.S. Pat. Nos. 3,604,980 and 3,656,171 in which a dielectric planar surface has plated thereon a series of strips of electrically conductive material, each connected to a charging signal source. The “planar” charge electrode disclosed by Robertson differs from other prior art charge electrodes in that the conductive strips do not completely surround surround the drop streams. Rather, the charge planar charge elcetrodes disclosed by Robertson are offset to one side of the jets emitted by corresponding nozzles. The compact nature and form of planar charge electrodes may make them suitable for state of the art high-resolution continuous inkjet systems that incorporate a high number of very closely spaced nozzles. In this context, “high-resolution” refers to an effective native drop generator spacing on the order of 500 drops/inch (dpi) or greater.
Prior art electrostatic continuous inkjet systems have mostly employed either a single inkjet nozzle, or a single row of nozzles. Attempts have been made in the prior art to increase the resolution of such devices. In U.S. Pat. No. 3,560,641, Taylor et al. discloses offsetting one or more rows of nozzles from one another in the direction of the nozzle array, in order to achieve a greater effective pixel density. Electrostatic continuous inkjet printing systems employing more than one row of inkjet nozzles are however typically, older systems with relatively large nozzle-to-nozzle separations. Further, these systems typically have relatively large inter-row separations usually on the order many hundreds of microns or even several millimeters. In U.S. Pat. No. 3,701,998, Mathis discloses a continuous inkjet apparatus in which twin rows of nozzles are separated from on another by 400 microns. This large separation is in part due to the fact that a drop deflection means comprising an electrically conductive strip is positioned between the two rows of continuous drop streams that are generated. In one embodiment of the “998” patent, the electrically conductive strip is grounded such that oppositely charged non-printing drops are guttered to opposing sides of the print-head. In U.S. Pat. No. 4,596,990, Hou discloses a dual row print-head wherein the jets are separated by 1-3 mm, and drops within each jet are separated by 152 um. Hou claims that the coulombic interactions between the adjacent jets are very small. Rows in the above patent are spaced by as much as 3 to 6 mm apart.
The spatial requirements of these prior art systems make them unsuitable for use in of state of the art high-resolution (i.e. 500 dpi or greater) electrostatic inkjet systems. These high-resolution systems require a large number of continues streams of very small drops to be formed and the drop to drop separation within a given stream must be much smaller than those of the prior art. Additionally, nozzle-to-nozzle separations, whether between jets in a given row, or additionally between rows in a multi-row system must conform to the small separations requirements of these high-resolutions. Different methods have been used to increase drop resolution. Micromachining manufacturing techniques have been employed to produce multiple rows of very closely spaced nozzles. Silverbrook has described in U.S. Pat. No. 5,892,524 a drop-on demand printer constructed using these micromachining techniques with nozzle-to-nozzle separations under 100 um. Further, an inkjet printer in which thermally stimulated drop separation is employed with nozzle-to-nozzle separations also under 100 um is described by Hawkins et. al. in U.S. Pat. No. 6,536,883, and also in U.S. Pat. No. 6,457,807. In these prior art systems, electrostatic charging and separation of drops is not employed.
Multi-jet continuous inkjet systems comprising electrostatic drop charging and separation architectures have proven themselves to be reliable and successfully capable of producing quality images at low to mid resolutions. However, high-resolution versions of these continuous inkjet printers, especially those requiring multiple rows of closely spaced nozzles, are however subject to undesirable electrostatic challenges when electrostatic drop charging and separation architectures are employed. In these high-resolution electrostatic systems, challenges including effective drop charging (i.e. charge coupling), as well as electrostatic nozzle-to-nozzle crosstalk and drop-to-drop electrostatic crosstalk, effects are further compounded and amplified by the spatial requirements imposed by a high-resolution architecture.
As previously stated, planar charge electrodes may be considered for such high-resolution printers because of their very compact nature. Additionally, the construction of planar charge electrodes is suited to standard thin film manufacturing techniques commonly used in the electronics industry. The planar charge electrodes may also be manufactured using a variety of other techniques including micromachining (MEMS). However, when closely spaced nozzle arrays as required by a high-resolution print-head are considered, effective charge coupling between any given charge electrode and its respective drop stream may not be enough to ensure minimal charge variations among the charged drops. The tight spatial requirements of high-resolution CIJ print-heads can lead to undesirable charge variations caused by indirect electrostatic effects between neighboring charge electrodes and a given drop stream. These charge variations will affect drops selected for printing, as well as drops selected for guttering within the given stream. Print drop charge variation will affect print quality by affecting the drop placement accuracy on the recording surface. Charge variation in drops not selected for printing, will affect the ability to effectively gutter and recycle the unprinted ink, impacting the reliability of the print-head. In the later case, the print-head length must typically be increased to accommodate a gutter that is long enough to capture non-printing drops that have not been fully charged. This longer print-head in turn amplifies any pointing errors associated with the print drops since they must now travel a longer distance to the recording surface. Poor print quality can thus offset the gains in higher print image resolution.
Poor print quality can occur when drops that are intended to remain uncharged, or are intended to have some specific amount of charge, actually have additional charge induced by the charge electrodes of adjacent or nearby nozzles. These adjacent or nearby charge electrodes may correspond to neighboring nozzles within a given row of nozzles or they may correspond to the neighboring nozzles within another row of nozzles. This “nozzle-to-nozzle” electrostatic crosstalk effect created by the associated charge electrodes of neighboring nozzles is particular prevalent when planar charge electrodes are employed. Unlike prior art charge electrodes that completely surrounded their associated drop streams, planar electrodes by their design, cannot easily do this. Consequently, the shielding effects that prior art tunnel charge electrodes provided between adjacent nozzles is not readily provided by planar electrodes, thus increasing the occurrence of nozzle-to-nozzle crosstalk effects.
In addition to nozzle-to-nozzle crosstalk effects, other undesired electrostatic crosstalk effects can manifest themselves within a high-resolution CIJ printer. The very high speed printing performance and small drop size requirements of current state of the art continuous inkjet recording systems require that the fluid streams be stimulated such that the resulting continuous streams of drops are made up of very closely spaced drops. In this situation, “drop-to-drop” electrostatic crosstalk can occur between consecutive drops emitted by a given nozzle. When drop-to-drop cross talk does occur within a given drop stream, a drop currently being charged may have its resulting charge adversely influenced by charge distortions created by the electric fields of preceding adjacent drops. These additional electric fields may prevent a specific drop from being charged with the correct charge level and thus lead to additional print quality issues.
Several approaches have been noted in the prior art to reduce drop-to-drop electrostatic crosstalk effects. In U.S. Pat. No. 3,562,757, Bischoff describes how the use of a number of “guard drops” between successive charged print drops acts as a shield to minimize the adverse cross-talk effects that the electric field of one charged drop has on the subsequent formation of another charged drop. A guard drop is a drop that is not used for printing, but which serves the sole function of separating a print drops within a drop stream, thereby reducing drop-to-drop crosstalk. Additionally, Bischoff states that this guard drop scheme further improves the aerodynamics of the drop trajectories. Specifically, Bischoff explains that every emitted drop leaves in its wake a region of turbulence that causes variability in the required trajectory of a following drop that enters the region of turbulence. When guard drops are employed, they are subsequently separated from the drops to be printed by the charge deflection plates. Therefore when the guard drops are separated, the spacing between the remaining “printable” drops is increased and the effects of turbulence are substantially reduced.
Needless to say, both the drop-to-drop crosstalk effects and the nozzle-to-nozzle crosstalk effects can further combine to compound the undesired charging effects that can occur in high-resolution multi-row continuous inkjet print-heads. In these systems the required charge level on a specific drop emitted from a given nozzle will be affected by charges on drops previously emitted in the drop stream of the given nozzle, as well as by the charges on drops previously and concurrently emitted in nearby nozzle drop streams.
The prior art has proposed several solutions to counter the undesired electrostatic charge effects created by the combined drop-to-drop and nozzle-to-nozzle crosstalk phenomenon. In European Patent Application No. 0104951, Paranjpe describes a dual row continuous inkjet system in which a pattern of charged guard drops are provided to isolate print drops from undesired electrostatic effects of other drops. In the “951” patent application, the guard drops in both rows are charged with a single polarity charge and the print drops are not charged or are slightly charged so as to print onto multiple positions on a recording media. A central deflection electrode that is positioned between the dual rows of nozzles deflects the single polarity guard drops outwardly. According to this approach, one or more guard drops are provided between print drops in each stream to reduce drop-to-drop crosstalk, and one or more guard drops are provided between print drops in each row to reduce nozzle-to-nozzle crosstalk. Paranjpe proposes various arrangements of guard drops and print drops.
Additionally, charge compensation schemes have further been proposed to minimize electrostatic crosstalk effects that give rise to non-optimal print drop placement. In U.S. Pat. No. 3,828,354, Hilton discloses such a charge compensation scheme. These approaches are suitable for low-density print-heads, but for state-of-the-art systems with high-resolutions and hundreds or thousands of nozzles per print-head, these methods become expensive. It is desirable to use less expensive digital circuitry to drive the many charge electrodes on a high-resolution print-head to avoid the cost associated with large numbers of analog drivers and associated systems controllers to determine the proper drive level.
As previously stated, drop trajectories can also be additionally adversely affected by aerodynamic effects. Although guard drop schemes may help in this regard, the prior art has taught additional methods to reduce these effects. In U.S. Pat. No. 3,596,275, Sweet discloses the utilization of a gas stream, such as air, to compensate for the aerodynamic drag on the ink drops. In U.S. Pat. No. 3,972,051 Lundquist et al discloses adjusting the airflow such that it remains laminar with a Reynolds number of less than 2300. Gas flow assist as disclosed by the prior art has for the most part been applied on a single nozzle or single row of nozzles.
Clearly, producing a reliable, high quality high-resolution electrostatic CIJ print-head requires consistent drop charge coupling as well as over coming the aforementioned drop-to-drop and nozzle-to-nozzle crosstalk effects and the aerodynamic effects. Additionally, an effective deflection field is required to minimize the time of flight of emitted drops. Reducing the drop time-of-flight minimizes the amount of time that any remaining crosstalk and aerodynamic effects can have on the trajectory of the drop, thus reducing print errors. In U.S. Pat. No. 4,395,716, Crean et al. discloses a bipolar swathing inkjet printer, wherein the deflection field has an electrical field strength that is slightly less than the breakdown field strength of air for the environment in which the printer is to operate in.
As further print resolution improvements are required and nozzle structures are manufactured using micromachining methods, it is clear that there remain challenges when designing high-resolution continuous inkjet systems requiring superlative drop placement accuracy.
It would be advantageous to provide a multi-row electrostatic CIJ print-head with high native resolution of 500 dpi or greater. Such a high-resolution CIJ print-head should comprise a charging means operable for maintaining a high degree of charge coupling with each drop, while introducing a low amount of influence charging.
It would also be advantageous to provide such a high-resolution CIJ print-head with a charging means capable of also minimizing nozzle-to-nozzle and drop-to-drop crosstalk effects.
It would additionally be advantageous to provide such a high-resolution CIJ print-head with a gas system capable of maintaining a uniform laminar flow across each of the multi-rows of nozzles, thus minimizing the undesired aerodynamic effects among the drop streams emitted by the multi-rows of nozzles.
It would further be advantageous to provide such a high-resolution CIJ print-head with a drop deflection means capable of reducing the time of flight of charged drops and thus reducing the time for adverse electrostatic crosstalk and aerodynamic effects to alter the desired trajectory of the drops.
Finally, it would be advantageous that such a multi-row electrostatic CIJ print-head be produced by state-of-the art micromachining fabrication methods to produce a compact print-head suitable for print resolutions of 500 dpi or greater. Further, it would be advantageous for the print-head length in the direction of jetting be as short as possible so that the nozzle to recording surface distance is minimized, further reducing time-of-flight errors and drop placement errors due to the residual jet pointing error of the nozzles. Such a print-head should gutter non-printing drops in the shortest path possible.